Very strange home router problem [closed]

This sounds like an MTU problem.

There's likely something between you and those sites that doesn't support the typical 1500 byte MTU, and on top of that probably a firewall blocking the ICMP packets that are used for "Path MTU Discovery", so your end can't tell that the normal MTU can't be used.

Try a traceroute, and then for each hop in turn, try sending a large ping packet (1492 bytes) and see if any of those hops refuse to return the packet.

EDIT - your tcpdump output shows that your end is still trying to initiate TCP's "three-way handshake" because the SYN bit is sent in the packets from your end. However the packets coming back from Adobe appear to be truncated or malformed. That's pretty weird, because there shouldn't be any payload in the packets, just the far end's SYN response. I'd need to see a full dump (including the -X option) of just those first 4 or so packets to know more.

EDIT2 - based on your detailed tcpdumps I believe that your router is corrupting the TCP response from some sites. The best way to test this is to borrow another brand of router.

Plug one of your computers directly into your internet connection and let it get all it's network settings from your ISP. If you can't access the sites then it's an ISP issue, if you can then it's a router issue and you can go from there.

You can try a traceroute, and see how far your packets are getting. If they're stopping at your router, it's probably a problem there. If they go farther, you might want to get in touch with your ISP.

Reading your question again, you say you can ping the servers successfully, so you might not see anything abnormal on the traceroute...

I definitely agree with the notion that basic symptoms of this problem sounds like it is related to a PATH MTU problem. There are other possibilities, but that is the most likely place to start.

Given the prominence of the sites you mention and presumably the extended period of time that this has been occurring for, it seems kind of unlikely it is a problem within the ISP's network......although given the traceroute result shown in the question, the path depth and total latency doesn't shine very well on your ISP. Generally speaking, any decent ISP should get you to any major/prominent web property (within the USA) in something [well] under 120ms...but I digress.

Using traceroute and ping to diagnose the problem as others have mentioned is very helpful, but it is far from a definite tool solution given the possibility/likelihood of ICMP blocking/filtering in various locations. And, because of this, except in the hands of a skilled analyst it is pretty hard to tell the difference between specific problems & firewalls messing with ICMP.

The best way to rule out an MTU problem is to start by reducing the MTU of the Ethernet interface in one of the computers that is having the problem. See the procedure located here for MAC systems since you mentioned you have a MAC computer.

If you start lowering your interface MTU as the process describes in steps of say 100 bytes at a time and checking functionality starting from from 1400 down to 500 bytes.....if the problem suddenly goes away at one of the steps, then you definitely have a path MTU problem for sure. If dropping down to 500 as a minimum doesn't solve it, then it is not a path MTU problem and you can move on to investigating other possibilities (after you switch your MTU back up to where it started...which was probably 1500 bytes).

I've fixed the problem now and in the end the fix was deliciously simple. I logged a support call with my ISP (PlusNet) and they sent me a link to a forum post explaining that this problem is a bug in my router's firmware. The fix was simply to set the router's Internet connection MTU to 1500 (the default is 1400) so that it matches the router's LAN side MTU.

Thanks to everyone who offered help and advice. I'm going to accept Alnitak's answer simply because he/she stuck with me on this and kept coming back with more advice and things to try.